An adapter includes an optical connector accommodation part having a first cavity in which a first optical connector having a first front end portion is to be accommodated, and a second cavity in which a second optical connector having a second front end portion is to be accommodated, and a spacer having a first surface configured to contact the first front end portion, a second surface configured to contact the second front end portion, and a light transmission part configured to enable a light beam to pass therethrough. The spacer being arranged between the first cavity and the second cavity. At a state where the first front end portion is contacted to the first surface and the second front end portion is contacted to the second surface, the first optical interface part and the second optical interface part face each other at a predetermined interval.